Electron beam device



ELECTRON BEAM DEVICE .Filed April 14, 193s 2 Sheets-Sheet 1 f ma va; maf

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of. B: 279 1940 G. F. METCALF ELECTRON BEAM DEVICE Filed April 14, 1938 sheets-sheet 2 Patented Feb. 27, 1940 ELEc'raoN BEAM nevica George F. Metcalf, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application April 14, 1938, Serial o. 201,954

(Cl. 25o-27) 'I'his is a continuation-impart of application ity modulates the beam without producing ap- 1 Claim.

Serial Number 153,602, led July 14, 1937, by

W. C. Hahn and G. F. Metcalf.

The present invention relates to electron beam devices and, while not limited thereto, is especially useful in connection with velocity modulation devices -such as are described in the aforesaid application.

In devices in which it is necessary to employ a concentrated electron beam and to cause such a beam to traverse a considerable space, there is a tendency for progressive spreading of the beam to occur. This is due primarily to the mutually repulsive forces exerted by the electrons on one another.

-It is one object of my present invention to provide improved electrostatic means for maintaining .a beam of electrons or other charged particles in focus irrespective of the length of thepath which it traverses.

'I'he features of novelty which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself will be described in connection with a velocity modulation device of the type disclosed in the aforesaid application S. N. 153,602. For a' better understanding of the structure involved reference may be had to the drawings, in which Fig. 1 illustrates an electrode combination suitable for producing velocity modulation of an electron beam; Fig. 2 illustrates dlagrammatically a velocity modulation discharge device embodying my invention; Figs. 3 to 5 are representations useful in explaining the invention; Fig. 6 illustrates diagrammatically a modification of the device of Fig. 2 and Figs. 7 and 8 show possible modifications of the beam focusing means.

In the drawings above referred to, I have indicated certain voltage ranges as being suitable for 40 the operation of the various electrodes. It should be understood, however, that the values given are exemplary only, and that they may be varied within wide limits, even to the extent of changing their order of magnitude.

In the parent application mentioned above, it is pointed out that an electron beam may be modulated in two Ways-either as to velocity or as to charge density. In conventional electronic devices these two types of modulation are inevitably combined. In the novel devices described in the said application, however, a separation of the modulating processes is eiected, thus improving the operation of the devices at ultrashort wave lengths. This is accomplished by the use of a control electrode structure which velocpreciable charge density variations inthe vicinity of the control electrode structure. The velocity modulation this produced is subsequently and independently converted into charge density modulation of a higher order of magnitude, and the charge density modulated beam is then caused to excite an output circuit.

In Fig. 1 there are illustrated the basic elements of one type of electrode system suitable 10 forproducing velocity modulation. These elements provide a modulating space which is substantially shielded from *the electron beam source or cathode. By virtue of such shielding, current or potential variations which occur in the modu- 15 latingspace have no tendency to effect changes in the cathode emission or to produce charge density variations in the space as a result of such changes. y A

A modulating space as specied inthe foregoing may comprise, for example, a space which is arranged to be traversed by an electron beam, and the entrance and exit boundaries of which are maintained at fixed potentials with respect to one another. Thus, in the particular arrangement shown in Fig. 1, a modulating space is provided between two apertured conducting barriers or diaphragms I0 and I I which are. electrically connected to one another and maintained at a definite potential above ground. The chamber thus dened is, in use, traversed by an electron beam I3 which may enter through an opening I5 in the diaphragm I0 and leave through a corresponding opening I6 in the diaphragm II.

In order to produce velocity modulation of the y beam, the potential level `of an intermediate region of the space should be cyclically raised and lowered with respect to the boundary potentials at such a rate that the velocity of any given electron in the beam is similarly affected as the electron approaches andrecedes from such intermediate region. In connection with the particular structure of Fig. 1, this may be accomplished, for example, by the use of a control electrode in the form of a tube I9 which is posi- 45 tioned between the diaphragms ID and II and spaced slightly from each of them. If the potential of this electrode is caused to vary above and below that of the diaphragmsv with the proper periodicity, as by connection to a source of cyclically varying control voltage 2|, effective velocity modulation of the transmitted beam may be obtained.

For reasons which need not be elaborated here, velocity modulation by the means described may be accomplished without appreciable power loss in the control circuit. The utility of the arrangement lies partly in the fact that slight velocity variations, produced without substantial power loss, may be subsequently converted into much greater magnitudes of charge density variation, thus producing amplication effects.

In Fig. 2 there is shown diagrammatically a complete velocity modulation amplier in which the particular improvementwhich comprises my present invention is incorporated. In the construction shown there is provided .a discharge device which comprises a sealed envelope 25 which is indicated in dotted outline. Within the envelope there is provided means vfor developingv an electron beam of substantially constant average intensity and velocity. Such means may include any known type of electron gun, and that illustrated constitutes only one example of many possible constructions. In the arrangement shown, the electron source comprises a thermionic cathode including a filamentary heater 26 andan enclosing electron emissive cylinder`21. Surrounding the cathode as a whole there is provided a focusing electrode in the form of a conducting tubular member 29, which is supported directly by an insulating bushing 30 and indirectly by means of a second tubular member 3|. 'I'he latter member is secured to a transverse barrier 32 which constitutes one boundary of a velocity modulating space.

In the use of the device, the illamentary heater 26 is energized by means of a suitable energy source, such as a battery 33, connecting with the heater. The focusing electrode 29 'is maintained at cathode potential or at a potential which is slightly negativewith respect to the cathode so as to concentrate the electrons emitted from the cathode surface into a beam of generally cylindrical outline. (A. battery 34 has been indicated as a source of biasing potential.) Such a beam may be given a desired velocity by impressing an appropriate potential .between the cathode and the transverse diaphragm.32, as by means of a battery 35.

Somewhat spaced from the conducting member of diaphragm 32 there is provided a second diaphragm 31 which is maintained at an identical potential with the first diaphragm by being electrically connected thereto as by a conductor 38. These diaphragms are respectively provided with central apertures 39 and 40 and form a modulating chamber which is shielded from the beam source or cathode. Within this chamber velocity modulation of the beam may b e accomplished by means of a tubular control electrode 42 corresponding to the similar element described in connection with Fig. 1.

In the use of the device as an amplifier or detector, the potential of the electrode 42 may be alternately raised and lowered by impressing thereon a cyclically varying control potential. This may be derived, for example, from an antenna 43 feeding into a tuned circuit which comprises a condenser 44 and an inductance 45.

Assuming the device to be in normal operation, the electron beam issuing from the opening Will be velocity modulated. That is,` it will be characterized by successive variations in electron velocity from point to Vpoint along the beam. The degree of modulation may be extremely slight if only weak control potentials are available but it may be changed into charge density modulation of a considerably higher order of magnitude by conversion means now to be described.

In Fig. 3 the electron beam is shown as it is assumed to issue from the modulating space. It will be noted that at this time it comprises alternate spaced groups of fast electrons and slow electrons, the former being designated by black dots and the latter by light dots. l

In Fig. 4 the condition of the same beam at a somewhat later period is illustrated. As shown, the fast electrons have caught up with the slow electrons so that definite irregularities in charge density distributiomhave been produced, corresponding to a high degree of charge density modulation. 'I'he change which has taken place is from its nature one that requires only the elapse of time and the absence of extraneous infiuences which might tend adversely to ailect` conditions within the beam. 'I'hese requirements may be fuliilled by permitting the velocity modulated -beam to pass through a relatively long shielded space or drift space which is iree of any but static potentials.

The appropriate length of the drift space is determined by various factors including the beam velocity, the beam density and dimensions, and the nature of the external electrostatic or magnetic fields acting on the beam. Generally speaking, the time required to realize maximum charge density modulation from a given velocity modulation is on the order of, but somewhat less than, the duration of a quarterl cycle oi the control potential divided by the per unit velocity modulation. The best length of the drift tube is this time multiplied by the average velocity u of the beam through the tube and may be determined experimentally for a given set of fixed electrical conditions. It is not necessary, as a practical matter, that the drift tube be of such length as to permit maximum conversion oi' the velocity modulation into charge density modulation. A considerably smaller degree of conversion will be eective to produce substantial amplification of the input potentialif that is the desired end result.

'I'he use oi a drift space of appreciable length makes it necessary to take into consideration the inherent tendency of an electron beam to become defocused or spread by virtue of the inherent dispersing effect of its own charges. This difficulty, insofar as it occurs in connection with the use of a drift space, may be substantially overcome by a drift tub construction as illustrated in Fig. 2. In this case the drift space is surrounded by a plurality of electrically separate similar conducting members B0 to 54 of which alternate members are electrically connected together as shown. The end section is preferably operated at a relatively low potential (say, 100 volts negative with respect to the boundary diaphragm 31) so that the average beam velocity is reduced as the beam enters the drift tube structure. By operating alternate ones ofthe drift tube elements at this potential while maintaining the remaining elements at a relatively higher potential (say, 200 volts positive) a repetitive focusing of the beam may be produced as the beam traverses the drift space. Batteries 56 and 51 may be used to maintain the potential relationship'rei'erred to.

'I'he focusing eilect specified may best be understood by reference to Fig. 5 in which are shown the drift tube elements ll, 52 and 53 together with a typical iield -plot of the equipotential surfaces existing in the spaces between them.

The boundaries of the electron beam are indicated by the dotted lines 59.

In accordance with known principles of electron optics the various rays of the electron beam,

as they leave the field-free space Within the cylinder 5 l are subjected to a force perpendicular to the equipotential surfaces. Ay vector typifying such a force acting on the outer boundary of the beam is indicated at F. It will be seen that this force has a component F' tending to retard the beam and another component F tending to cause outward dispersion of the beam. These components prevail until after the beam has passed the plane A which is midway between the elements 5| and 52. Thereafter, the forces acting on the beam are such as are indicated by the vector G and its components G' and G". Of these, it will be seen that the transverse component G" is in such a direction as to tend to re-concentrate or focus the beam. From the -foregoing discussion it might be thought that the two eifects just described would precisely offset one another. 'I'hat this is not the case is due to the fact that the beam is retarded during its passage between the elements 5| and 52 as a result of the potential difference which exists between them. Consequently the average velocity of the beam in passing between Ythe plane A and the boundary of the element 52 is less than that of the beam as it passes from the boundary of the element 5| to the plane A. As a result the beam is within the focusing field to the right of the plane A for a longer period than it is influenced by the focusing field to the left of the plane, and a net focusing eifect is obtained.

A similar effect occurs in the space between the electrode 52 and the electrode 53, where acceleration of the beam takes place. Here the focusing effect is predominant as long as the beam is at the left of the plane B while the defocusing effect prevails to the right of such plane. Due to the different times during which these effects act, the focusing tendency exceeds the defoousing tendency. fr

It is clear that focusing of the type above referred to can be repeated indefinitely by providing a succession of electrode elements charged alternately to high and low potentials. At the same time the average velocity of the beam will not be greatly affected by such an electrode system due to the fact that the accelerating effect of one electrode is offset by the retarding effect of the following electrode. By a judicious choice of geometrical and electrical relationships the drift space may be made of as great length as desired without the occurrence of undue spreading of the beam.

After passage of the electron beam through the drift space with resultant conversion of its velocity modulation into charge density modulation, the energy of the charge density modulated beam may be utilized in an external circuit. This may be accomplished, for example, by surrounding the beam with a further tubular electrode 60 placed in a shielded space provided by the two boundary diaphragms 6I and 62.

The electrode 69 is obviously coupled to thel beam so as to be affected by the variationswhich occur in it. As explained in the aforesaid application S. N. 153,602, if the electrode 69 is properly proportioned it may be caused to take energy from the beam in a manner which is, in a sense, the inverse of that which characterizes the action of the input electrode 42. This energy may be imparted to a tuned output circuit comprising the combination of a condenser 64 and an inductance 65, these being coupled to output terminals 66. After passage through the electrode 60 the beam may be collected by an anode 69 which is maintained at an appropriate positive potential.

A modified application of the invention is shown in Fig. 6 in which all the electrode elements constitute parts of the beam focusing system. In this arrangement the boundary diaphragms 32 and 31 of Fig. 1 are replaced by cylinders 'I9 and 12 and the boundary diaphragms 6I and 62 are replaced by cylinders 18 and 89. The other elements are in substantially the same form as previously described, so that the electrode system as a whole comprises a series of similar cylinders numbered 'l0 to 80 inclusive. Alternate cylinders are maintained at a relatively high potential, for example, by connection to a-battery 82, while the remaining cylinders are maintained at a relatively lower potential by connection to a battery 83. In

this way all the electrodes are caused to assist in` the focusing action.

The particular form of focusing electrode is in no way essential to the purposes of the invention. In Fig. 7 I have shown an arrangement of drift tube elements which may, in some cases, advantageously replace that of Fig. 2. In this embodiment the focusing elements are constituted of a series of hollow truncated cones, to 94 inclusive, positioned in end-to-end relationship so that alternate cones are oppositely directed. These cones are preferably -maintained at alternately high and low potentials such that the electron beam (indicated by the dotted lines 96) is caused to focus in the regions where the cone dimensions are smallest. As a result of this disposition the dimensions of the cones are greatest at points where maximum defocusing of the beam occurs. Consequently, even at such points, the outer elements of the beam do not approach the electrodes very closely and the collection of electrons is minimized.

In Fig. 8, a further variation is shown which takes into consideration the fact that even with a focusing system such as that described there will be certain components of the beam which are not maintained in focus. These components, which comprise electrons having a velocity apprecialy different from the average beam velocity, are always present either due to random effects or to the velocity modulation impressed on the beam by the control electrode structure. Assuming that the focusing system is adjusted to have the desired effect on electrons travelling at the average beam velocity, there will be at least some electrons of different velocity which will not be properly controlled and which disperse outwardly toward the electrode surfaces.

By providing a series of focusing elements 98 to |92 which progressively increase in` diameter as one proceeds along the beam as shown in Fig. 8, one can minimize the number of electrons collected. The greater diameter of the elements to the right permits a greater cross-section of the beam without appreciable electron collection. The variation in the shape of the electrostatic eld produced by increasing the diameter of the 'focusing elements may be offset by simultaneously increasing their length slightly so that the elements of greatest diameter are also of greatest length. An arrangement of this kind may be used, for example, in an electrode system such as that shown in Fig. 6.

While I have described my invention especially in connection with velocity modulation devices, it

of the above .numbered patent requiring correction as follows:

bodiments it will be understood lby those skilled in the art that many modifications may be made Without departing from 'the invention. I, therefore, aim in the appended claim to cover all such equivalent forms as come within the true spirit and scope o1' the disclosure.

Patent-No. 2,192,059.

alcance What I claim as new and desire to secure by Letters Patent oi' the United States, is:

In a discharge device, means' for producing an electron beam, means providing an elongated space to be traversed by the beam, a series o1' hollow irustro-conlcal members, each arranged to surround the path of the beam, alternate ones of said members being oppositely directed and means for causing repetitive focusing of the beam inthe regions of least divergence of the conical members, said last-named 'means including a circuit --ior applying a relatively high potential to alter-,-

nat'e ones of said members and a relatively lower potential to the remaining ones of said members.

. GEORGE F. METCALF.

CERTIFICATE oF CORRECTION.

'February 27', 19110;

GEORGE F. 'METCALR It is hereby certified that "error appears in the printed specification Page 5, first column, line 5h? for the word "focusing" read defocusing/j same page, second column,`A line Y[L9-50, for ."apprecialy" read appreciably;

and that the said Letters Patent should be read withthis correction therein that the same may conformI to the record of the casein the Patent Office.

signed and sealed this 9th day`of April, A. D. 19ho.

-(Seal) Henryv Vian Arsdale, Acting Commissioner of Patents.

- v CERTIFICATE oF'CORRECTION. l Patent No.` 2,192;0I L9. v' I J j'February 2T, l9hO`.

vGEORGE F. 'METCALR It is hereby certified that "error appears .in the printed specification of the above .numbered patent requiring correction as follows: Page 5 first column, line 5h, for the word "focusing" read defocusing/gr same page, second column,v lir1e-L'L95O-, for "apprecialy" read appl'eciably;4 and that the said Letters Patent should be read with`this correction therein that the same may conform to the recQrd'of the case. in they Patent Office.

signed and saale@ @159th Acmy'nf April, A. D.- 19mL Henry' V'fan Arsdale, (Seal) Acting Commissioner of Patents. 

